Abstract
Expressions for the mass transfer resistances of all the physical domains composing the air-gap membrane distillation (AGMD) and direct contact membrane distillation (DCMD) processes are developed and their absolute and relative effects are evaluated to improve the process understanding and identify promising ways for its improvement. The resistances are computed based on the authors’ two-dimensional conjugate model in which a simultaneous numerical solution of the momentum, energy and diffusion equations of the feed and cold solutions have been carried out, and the results of which were validated in comparison with available experimental results. Some of the main conclusions are that: (1) the use and examination of process domain mass transfer resistances is indeed an effective method for understanding the process and identifying ways to improve it, (2) the air/vapor gap dominates the mass transfer resistances of the AGMD domains, and while increasing the air/vapor gap width reduces the parasitic heat transfer by conduction, increasing the width beyond 2
mm has thus not improved the process thermal efficiency, (3) the hot solution inlet temperature and the air gap width have by far the strongest effect on the domain mass transfer resistance, mainly as a consequence of their effect on the air/vapor gap mass transfer resistance, (4) the inlet velocities of the hot and cold solutions have a small effect in AGMD, where the effect of the hot solution velocity is the higher one, (5) the concentration of the solution has a slight effect on the process, (6) the material used for the membrane should have a small thermal conductivity for a more efficient MD process and (7) efforts to minimize the mass transfer resistance of the cold solution will have a relatively small effect on the permeate flux.